Design of High-frequency Fast-rise Pulse Modulators for Lifetime Testing of Dielectrics

Penetration of power electronics in the grid has produced a new species of stresses, characterized by fast-rising pulsed waveforms with microsecond rise times repeating at several tens of kilohertz. Analyzing their impact on existing and future insulation systems requires pulse modulators, most often with pulse transformers (PTs), to perform aging and breakdown tests. PT design for klystron loads has been studied extensively albeit for either low repetition rates or short pulse durations. However, capacitive dielectric loads impose additional complex constraints on optimizing leakage ( ${L}_{\sigma }$ ) and parasitic capacitance ( ${C}_{d}$ ) in order to minimize rise time ( ${T}_{r}$ ) and overshoot ( $V_{\text {pk}}$ ). Ensuring consistent output pulse shape is crucial since breakdown is sensitive to voltage magnitude. This article discusses these challenges through the design procedure of a modulator prototype capable of producing bipolar pulses up to 14 kV with rise times $ < 2 ~\mu \text{s}$ at frequencies between 10 and 50 kHz. Major challenges, especially core selection, winding design, PT parasitic optimization, breakdown detection, and failure modes, are highlighted. A new PQR equation is derived to model modulators with capacitive loads. Finally, the output pulses are applied across oil-impregnated paper samples to generate statistics on insulation breakdown strength and lifetime at 10 and 50 kHz. Results illustrate a reduction in lifetime and breakdown strength at 50 kHz. This is possibly due to the nonhomogenous distribution of dielectric losses within the oil-paper leading to local hotspots and eventual thermal breakdown. Furthermore, a critical field ${F}_{c} = {21}$ kV/mm is found below which the slope of the lifeline decreases dramatically, thereby indicating a shift in the aging mechanism. Potential reasons for this phenomenon are also discussed.